6 hours of sleep.

Six hours of sleep sits just below the NSF-recommended minimum of 7 hours. About 30% of adults regularly sleep 6 hours or less, and most of them report feeling "fine" — but when tested under controlled conditions, 6-hour sleepers show measurable declines in sustained attention, reaction time, and working memory after just two weeks of this schedule, even when they don\u2019t notice it themselves. Six hours is roughly 4 full sleep cycles for someone with a 90-minute cycle, or 4.8 cycles for someone with a 75-minute cycle. The interesting quirk: because 6 hours = 360 minutes, and most people\u2019s cycles are between 75 and 115 minutes, the remainder rarely lands you at a clean cycle boundary. That\u2019s why 6-hour sleepers often wake feeling groggy — the alarm is usually interrupting the middle of a late cycle. The key intervention if you\u2019re targeting 6 hours is to pick a bedtime that lands your wake time at a cycle boundary rather than a fixed clock time. A 6-hour sleeper with a 90-minute cycle can use a 6-hour window or extend slightly to 6 hours 30 minutes (which is 4.33 cycles — still groggy) or 6 hours 45 minutes (which is 4.5 cycles — also groggy) or 7 hours (which is 4.67 cycles, better but still imperfect). The cleanest 6-hour alternatives are 5 hours 50 minutes (3 × 100 + 80 ≈ 3.5 cycles) or 6 hours 15 minutes (5 × 75 = 375 min = 6.25 hours, 5 cycles for a 75-min-cycle sleeper). Six hours works best for a minority of adults who carry the "short sleeper" variant of the DEC2 gene — they can operate indefinitely on 6 hours without any of the debt accumulation. If you\u2019re not sure whether you\u2019re one of them, assume you\u2019re not: the gene is present in under 3% of the population.